Glow plug

Abstract

A glow plug comprising: a ceramic heater which houses a heating element defined herein in a tip end side of said ceramic heater, and which elongates in an axial direction; a cylindrical metal shell which has a shaft hole, which houses a rear end portion of said ceramic heater in said shaft hole, and which holds said ceramic heater in said shaft hole directly or via another member; and a center pole including a center-pole rear end portion as defined herein and a center-pole tip end portion as defined herein, said ceramic heater being mechanically rigidly joined to said heater connecting portion, wherein a stress releasing portion in which a diameter is smallest in an area between said heater connecting portion and said center-pole rear end portion is formed in said center pole.

1. A glow plug comprising:
 a ceramic heater including a heating element provided in a tip wend side thereof, said heating element being capable of generating heat upon energization; 
 a cylindrical metal shell which has a shaft hole, which houses a rear end portion of said ceramic heater in said shaft hole, and which holds said ceramic heater in said shaft hole directly or via another member; and 
 a coaxial center pole including:
 a terminal portion provided in a rear end of said center pole, said terminal portion being supplied with electric power from outside directly or via another member; 
 a heater connecting portion having a hole provided in a tip end of said center pole, a rear end portion of said ceramic heater being mechanically rigidly fitted into said hole; and 
 a stress releasing portion having the smallest diameter in an area between said heater connecting portion and said terminal portion. 
 
 
   
   
     2. The glow plug as claimed in  claim 1 , wherein said heater connecting portion comprises:
 a cylindrical member; and 
 a center-pole filling portion fitted into said cylindrical member. 
 
   
   
     3. The glow plug as claimed in  claim 1 , wherein the center pole comprises a center-pole front large-diameter portion provided in an area between said heater connecting portion and said stress releasing portion, said center-pole front large-diameter portion having a larger diameter than that of said stress releasing portion. 
   
   
     4. The glow plug as claimed in  claim 2 , wherein said center pole comprises:
 a center-pole front large-diameter portion having a larger diameter than that of said center-pole fitting portion, and 
 a positioning end face, against which a rear end face of said cylindrical member buffs, provided between said center-pole front large-diameter portion and said center-pole fitting portion. 
 
   
   
     5. The glow plug as claimed in  claim 4 , wherein said cylindrical member includes a portion which is adjacent to said center-pole front large-diameter portion, the portion having an outer diameter substantially equal to that of said center-pole front large-diameter portion. 
   
   
     6. The glow plug as claimed in  claim 2  wherein an outer diameter of said cylindrical member is substantially uniform along an axial direction. 
   
   
     7. The glow plug as claimed in  claim 1 , wherein said stress releasing portion is provided on a tip end side with respect to a midpoint in an axial direction of said center pole. 
   
   
     8. The glow plug as claimed in  claim 1 , wherein said center pole comprises a tip end-oriented inclined face which is adjacent to a rear end side of said stress releasing portion, and in which a diameter is more increased as further advancing toward a rear end of an axial direction. 
   
   
     9. The glow plug as claimed in  claim 1 , wherein said center pole comprises a rear end-oriented inclined face which is adjacent to a tip end side of said stress releasing portion, and in which a diameter is more increased as further advancing toward a tip end of an axial direction. 
   
   
     10. The glow plug as claimed in  claim 1 , wherein said center pole comprises: a tip end-oriented inclined face which is adjacent to a rear end side of said stress releasing portion, and in which a diameter is more increased as further advancing toward a rear end of an axial direction; and a rear end-oriented inclined face which is adjacent to a tip end side of said stress releasing portion, and in which a diameter is more increased as further advancing toward a tip end of an axial direction. 
   
   
     11. The glow plug as claimed in  claim 10 , wherein, when an acute angle formed by said tip end-oriented inclined face of said center pole with respect to an axis of the center pole is α, and
 an acute angle formed by said rear end-oriented inclined face of said center pole with respect to said axis is β, 
 a relationship of
   α<β 
 
 
     is satisfied. 
   
   
     12. The glow plug as claimed in  claim 8 , wherein said glow plug comprises an O-ring fitted between an outer circumferential face of said center pole and an inner circumferential face of said metal shell, and
 said  0 -ring is provided on a rear end side with respect to said tip end-oriented inclined face.